Page 115 of Scott Persons book Growing and Marketing Ginseng, Goldenseal, and Other Woodland medicinals I think you will see that the problem is Alternaria blight. It is caused by a fungus and spread by spores. By the look of your bed, you are growing many plants in close proximity to one another, as in Woods cultivated. If you are, you will need to invest in a back pack sprayer and some fungicides and adopt a regular regime of spraying over the course of your crop's life. Probably every week! Why are watering? Are things really that dry? Moisture will provide fungus with the ideal environment to thrive. You should maybe get the book.
I would be interested in knowing how long you have had this problem and how bad the infection has become. Is it killing the plants?
If it is what I am guessing, it can spread quickly effecting your other plants. It usually does not harm the root. However, it hangs around in other dead litter on the forest floor and when conditions are right (Hot humid and moist), it will emerge the next year and subsequent years. If it effects plants that are too young or weak to set a bud for the following year, it can kill the plant. Good luck.